The extracellular hemoglobin of annelids and tube worms are multisubunit proteins of up to 200 polypeptides and molecular masses to at least 3,900 kDa. They differ from all other Hbs in having O2 binding chains and "linker" chains. The linker chain L1 of the hemoglobin of Lumbricus terrestris contains a 38-39 residue segment with a repeating pattern of cysteinyl residues: (CysX6)3 -CysX5-CysX10-Cys. This pattern, not present in any globin sequence, corresponds exactly tao the cysteine rich repeats of the ligand binding domain of the low density lipoprotein (LDL) receptor of man and Xenopus laevis. The disulfide connectivity of these domains has not been determined in the LDL receptor. The determination of the disulfide bonds in the linker chains is therefore an important first step in the understanding of the interaction between apolipoprotein E and the LDL receptor. Linker chain L1 was digested with a number of enzymes and the resulting peptide mixtures were analysed by matrix-assisted laser desorption mass spectrometry. Due to the huge number of possible disulfide bridges, the assignment of peaks to disulfide bridges containing peptides was extremely difficult. Therefore HPLC fractionation of peptide mixtures were carried out. For the identification of disulfide containing peptides fractions of interest further enzymatic and chemical reactions are performed.